1. Field of the Invention
The present invention concerns a method to separate a magnetic resonance (MR) system-dependent phase influence from a subject-dependent phase influence in phase values of an MR phase image data set, and an MR system for implementing such a method.
2. Description of the Prior Art
The phase information of a magnetic resonance signal, which describes the attitude (orientation) of the magnetization perpendicular to the B0 field direction, can be used in many ways in magnetic resonance tomography (MRT). For example, the phase information contained in the MR signal can be used to separate fat and aqueous tissue, for flow measurement, in susceptibility-weighted MRT and for temperature determination. In a method known as thermotherapy, the temperature in tumor cells is increased in a targeted manner in order to kill the tumor cells or to make these sensitive to accompanying therapy measures. For example, the tissue heating can take place by focused ultrasound or with the use of lasers. In order to not destroy healthy tissue due to the increased temperature, temperature monitoring of the heated tissue is required. Certain MR parameters—such as the chemical shift, the T1 relaxation time or the diffusion constant for non-invasive temperature measurement—can be used in addition to an invasive temperature measurement with temperature probes placed in the heated tissue.
In the case of temperature monitoring based on the temperature dependency of the chemical shift (PRF), the resonance frequency altered by the temperature change is detected in an image point in an altered phase position. Only temperature changes can be shown in temperature imaging based on chemical shift, for example by taking the difference of two phase image data sets that were acquired at different temperatures. The phase image data set acquired at a known capture temperature serves as a reference data set from which the subsequent phase image data sets are subtracted. These methods operating with reference data sets have the disadvantage that movements of the examination subject between the acquisition of the two data sets, or other external interferences, lead to phase changes that are incorrectly interpreted as temperature changes. Furthermore, the B0 field constancy over time and current drift in the shim coils play a role, since these also lead to phase changes in the detected signal that can likewise incorrectly be identified as temperature changes. In addition to these methods with reference image data sets, there are techniques known as reference-less (reference-free) methods in which a temperature is concluded only from the measured phase values. These methods have the disadvantage that information must exist as to how the MR system-dependent background phase varies spatially across the image. The phase position in an image point is affected not only by the frequency of the magnetization in this image point but also by system components, for example the RF receiver or the demodulator.
In DE 10 2009 058 510.9 a method is described as to how the background phase in phase values of an MR phase image data set can be determined in a simple manner without using reference data sets. In such a method, however, the acquired phase image data set contains only signals of a single tissue type. For application to tissues of multiple tissue types with different frequencies (fat and aqueous, for example), the other tissue type (most often the fat) is suppressed by either only one tissue type being excited, or in that both tissue types are excited and the signals of the one tissue type are destroyed before the signal detection, so that the suppressed tissue type no longer provides any signal contribution to the MR signal. In image points at which low tissue proportions of the unsuppressed signal are contained, the method described in DE 10 2009 058 510.9 delivers only a low signal-to-noise ratio. Moreover, it is often difficult to completely suppress the signals of the unwanted tissue portion.
A further problem is that the susceptibility in the heated fat tissue changes in the case of non-invasive temperature imaging with the use of the proton resonance frequency (PRF) method, which in turn affects the phase values in the immediate vicinity, so the temperature information is adulterated. This subject-dependent phase influence is normally not separable from the system-dependent phase influence (due to B0 field fluctuations, for example).